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CdTe epitaxial layers grown by MOVPE on GaAs substrates
572
Journal of Crystal Growth 101 (1990) 572—578 North-Holland
HIGH RESOLUTION X-RAY DIFFRACTION STUDIES OF Cd~Hg1 ~Te/CdTe EPITAXIAL LAYERS GROWN BY MOVPE ON GaAs SUBSTRATES A.M. KEIR, A. GRAHAM, S.J. BARNETI’, J. GIESS, M.G. ASTLES and S.J.C. IRVINE Royal Signals and Radar Establishment, St. Andrews Road, Great Ma/oem, Worcs. WRJ4 3PS, UK
We have applied higb resolution X-ray diffractometry and topography techniques to investigate both the lateral uniformity and structural properties of Cd~Hg1_~Telayers grown by MOVPE onto CdTe buffer layers on GaAs. On samples — 1—2 cm square, maps of rocking curve width (/3) have shown values varying from 60 arc sec (comparable to the best reported) to over 1000 arc sec on the same slice, indicating the superior value of mapping over single point measurements on this material. A good correlation has been observed between rocking curve widths, lattice tilts and the density of pyramid-like surface defects, the last of which are also associated with an increased twin density. However, on rotating the sample about its surface normal, the 400 surface symmetric /3-value varies by up to an order of magnitude, indicating that lattice tilts play an important role in broadening the rocking curve. X-ray topography reveals large tilt boundaries in the CMT epilayer which correlate with the dislocation structure in the GaAs substrate.
1. Introduction Cd~Hg1_~Te(CMT) is an important material for detection of infra-red radiation in the 8—14 ,~mwaveband. To make possible focal-plane detector arrays of large area, epitaxial CMT with high quality and uniformity in both structure and composition is required. In our laboratories two prime candidate materials have emerged for the choice of substrate. CdTe has relatively poor structural quality and uniformity, is difficult to prepare with defect-free clean surfaces and is expensive. GaAs is much cheaper, it is of higher structural quality and uniformity and its surfaces can be prepared to a higher degree of cleanliness and perfection. The main disadvantage is the very high (15%) lattice mismatch with CMT compared to 0.3% for a CdTe substrate. Despite this latter point we have grown thick, high quality epitaxial CMT layers onto GaAs substrates and in this study these layers have been assessed using double crystal X-ray diffractometry where the FWHM (/3) has been used to describe their structural quality [1]. In assessing CMT epitaxial layers we have measured many /3-values on a two-dimensional array of points over each layer surface. Since many 0022-0248/90/$03.50 © 1990
—
CMT epilayers show large lateral variations in quality, these maps of /3 give a much more meaningful characterisation than a single point measurement. Uniformity of layers can be easily quantified from such maps. In addition, relationships have been investigated between /3-values and other properties, such as the density of surface defects, which are known to have a detrimental effect on device operation, and changes in crystal orientation, which have been found to be particularly large and anisotropic for CMT layers on GaAs [2]. In this study, a layer has been profiled by chemical etching to provide the X-ray data in three dimensions and investigate the origin of these important defects.
2. Experimental The double crystal X-ray rocking curves were measured on a Bede 300 diffractometer equipped with an X— Y sample stage. Control software for the X— Y stage, developed in-house, enabled rocking curve width (/3), peak height and relative angular displacement (e’) to be mapped automatically over an area of sample up to 75 x 75 mm. The X-ray source was a GX21 rotating anode
Elsevier Science Publishers B.V. (North-Holland)
A.M Keir eta!.
/ HRXRD
studies of Cd~ Hg
1 - ~Te/ CdTe epitaxial layers grown by MOVPE on GaAs
generator with a Cu target running at 2.8 kW. The reference crystal was 100 InSb and the 400 reflection was used for both the reference crystal and the sample in the + non-dispersive diffraction geometry. Beam size at the sample was 0.5 mm (X) by 1 mm (Y). The rocking curve data were mapped at 1.5 mm intervals in both X and Y directions. The theoretical /3 value for these diifraction conditions of the CdTe layers was calculated to be 14 arc sec and for Cd02Hg08Te was 23 arc sec. The X-ray extinction distance for the 400 reflection is 3.5 ~.tm for CdTe and 2.8 ~tm for Cd02Hg08Te layers. Lang topography [3] was used to examine the CMT layer and also the GaAs substrate after chemical etching had removed the CMT. The 511 reflection (extinction distance 5.5 ~iLm)was used for CMT epilayer and the 620 reflection for the GaAs substrate (extinction distance 9.7 ~tm). On all the diffraction data maps and topographs shown herein the arrow marked X indicates the direction of the incident X-ray beam projected onto the plane of the sample surface. The CMT layers were grown in a horizontal MOVPE reactor
using dimethylcadmium (Me2Cd) and diisopropyltelluride (Pr~Te)with a liquid Hg source at a vapour pressure of 0.01 atm. The layers were grown by the interdiffused multilayer process (IMP) whereby the gas flows are switched between conditions optimised for HgTe and CdTe growth [4]. 100 cycles of HgTe/CdTe interdiffused at the growth temp of 350°Cmade up the 16 p~mCMT layer analysed in this paper. Growth rates were 10.6 ~tm/h and 10.0 ~tm/h for HgTe and CdTe, respectively. The GaAs substrate was 2° off (100) towards (101). A CdTe buffer layer of 4 j.Lm thickness was grown between the substrate and CMT layer in order to prevent Ga diffusing into the CMT and to isolate the highly dislocated GaAs/CdTe interface [5]. Free etching of CMT layers was achieved using a 1% bromine/methanol etch (etch rate 4 ~.tm/min) and thicknesses were determined from etch calibration experiments based on masking and measuring step heights. Etch pit densities for the CMT layer were determined using the Polisar etch [6]. The total depth of layer removed between each set of X-ray measurements was 4 p~m.
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Fig. 1. Maps of the /3 values for the 400 reflection. Maps (a) and (b) are taken from the as-grown CMT epilayer surface and maps (c) and (d) of the CdTe buffer after chemical removal of the CMT epilayer. The maps cover an area 15 x 9 mm in the centre of the sample (the full extent of the sample was 23 x 12 mm).
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AM. Keir et a!.
/ HRXRD studies of Cd~,Hg1
~Te/ CdTe epitaxial layers grown by MOVPE on GaAs
3. Results and discussion
1000 D
Figs. la and lb show maps of rocking curve width (/3) for the sample before etching with the CMT layer at its as-grown thickness of 16 ~tm. These are recorded with the diffraction plane in orthogonal directions as indicated by the arrow, X. A well-defined region in which /3 is larger than for the remainder of the sample can be seen in both maps. In map (a) the minimum value of /3 is 91 arc sec and the maximum 225 arc sec. In map (b), /3 is greater at every corresponding point with a minimum of 152 arc sec and a maximum of 1230 arc sec. Figs. ic and id show the same area of the sample after chemical removal of the CMT to leave approximately 3 ~om of the CdTe buffer layer. Note that, although the scale is different to cover a range of higher /3 values, the general pattern is very similar to figs. la and lb. (The slightly higher values towards right and left are due to the CdTe layer being thinner after uneven etching.) Fig. 2 shows graphically the relationship between /3 values at equivalent points in the CdTe buffer layer and the CMT epilayer. Although the
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slope of the curve is much less than unity (indicating a large improvement of structural quality of the CMT compared to the CdTe buffer layer), the correlation observed between the /3 values clearly demonstrates the importance of buffer layer quality in influencing the final structural quality of the CMT epilayer. /3 values recorded at intermediate
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Fig. 3. Maps of relative angular position of the 400 Bragg peak. Again maps (a) and (b) are taken from the CMT sample surface and maps (c) and (d) of the CdTe buffer layer after removal of the CMT epilayer.
AM. Keir et aL
/ HRXRD
studies of Cd~Hg,- ,Te/ CdTe epitaxial layers grown by MOVPE on GaAs
4 ~tm depth intervals in the CMT layer follow a very similar pattern to that reported in previous papers [7,8] (for both orthogonal orientations of the diffraction plane). /3 decreases most rapidly in the few microns nearest the buffer layer and tends towards a constant value with increasing layer thickness. An estimate of the compositional uniformity of the CMT epilayer made by infra-red transmission measurements showed a maximum x variation of ±0.0015 over 1 cm. This could change the Bragg angle for the 400 reflection by only 1.5 arc sec and therefore should not contribute to significant broadening of the rocking curve. Typical sample curvature can move the Bragg peak by 200 arc sec over 15 mm of the sample and is expected to broaden rocking curves by 6 arc sec [7]. Simultaneous with recording the /3 values of fig. 1, the relative angular position (0’) of the 400 diffraction peak was recorded. Maps of this parameter are given in fig. 3. As in fig. 1, figs. 3a and 3b are for the CMT and figs. 3c and 3d for the CdTe buffer. These maps display the relative —
575
orientation of the layer (100) planes in the plane of diffraction. In figs. 3a and 3c the maximum change is 200 arc sec so lattice tilts may be confused with the effects of sample curvature. For the orthogonal diffraction in figs. 3b and 3d, the much larger values can be attributed to lattice tilt, the darkest regions representing a tilt in the (100) epilayer planes of almost 2° towards the (011) substrate planes. The very strong correspondence between figs. 3b and 3d shows that all the largescale tilt features present in the CMT layer originate in the CdTe buffer layer. It is interesting to note that although the /3 values are dramatically reduced on moving from the CdTe buffer layer to the CMT epilayer the relative lattice tilt (orientation) remains constant. Returning to fig. 1, note that the greatest rocking curve broadening occurs around the edge of the triangular tilted region in figs. lb and ld. This is due to the large change in orientation of the (100) planes across the area sampled by the X-ray beam at these points. Within the roughly triangular tilted region, /3 values are greater in both orthogonal directions. —
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Fig. 4. (a) 511 reflection topograph of the CMT layer; (b) 620 reflection topograph of the GaAs substrate taken through the CdTe buffer layer (note that these show the full sample area of 23 x 12 nun, a larger area than is covered by the rocking curve maps); (c) enlargement of the boxed region (i) of (b).
576
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/
HRXRD studies of Cd~,Hg
1 ~Te/ CdTe epitaxial layers grown by MO VPE on GaAs
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Fig. 4 (continued).
Etch pit density measurements also show higher values in this region. At the surface of the 16 j.tm CMT layer, values wereand approximately 3 X 2 in EPD the tilted region 6 x 106 cm2 l0~ cm for the majority of the sample surface. Since EPD values are believed to be proportional to dislocation density, this indicates a 5-fold increase in dislocation density for the tilted region with respect to the rest of the sample. —
Fig. 4a shows a Lang topograph of the CMT layer at 16 ~tm thick. Since all the large lattice tilts in the sample are sample perpendicular diffraction plane, the whole remainstointhecontrast but the diffracted X-rays from the tilted regions are displaced in the vertical plane. The large triangular tilted region can be seen clearly displaced towards the bottom of the figure. Laue diffraction patterns were taken to de-
A.M. Keir et aL
a
/
HRXRD studies of Cd~,,Hg
1— ,,Te/ CdTe epitaxial layers grown by MO VPE on GaAs
577
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Fig. 5. (a) Enlargement of the boxed region (ii) in fig. 4a. (b) Enlargement of the boxed region (ii) in fig. 4b.
termine the relative orientation of the epilayer with respect to the substrate, which was cut 2°off (100) towards the (101) lattice planes. It was found that, over the whole sample, the epilayer [011] direction lies parallel to the substrate [011] direction (both are, in fact, inclined at 1.4° to the sample surface). In the orthogonal direction, over most of the sample surface, epilayer and substrate [011] directions are inclined at 1.4° (in the (011) plane) to one another, ie the epilayer [011] direction lies in the plane of the sample surface. However, in the triangular region of high /3 values the epilayer [011] direction is tilted by a further 2° away from the substrate [011] direction. This observation relates to the amsotropic nature of the /3 values in that the largest values are observed when the relative lattice tilt between epilayer and substrate lies in the plane of diffraction. Fig. 4b shows a 620 reflection topograph of the GaAs substrate taken through the remains of the CdTe buffer layer. This topograph provides some important insight into the nature and origin of both the pyramid-like surface features and the large non-uniform lattice tilts measured in the CMT epilayer. Fig. ‘Ic shows an enlargement of
the boxed area (i) in fig. 4b. The white spots on this topograph correspond to the sites of the pyramid-like surface defects common to CMT epilayers. (In a previous paper [2] we reported on the link between these pyramids and twin density.) The majority of these spots can be seen to lie on the highly dislocated cell walls in the GaAs substrate rather than in the dislocation-sparse cell interior. Fig. 5 shows enlargements of the boxed regions (ii) of figs. 4a and 4b. Note that the horizontal scales do not match due to the different reflections used for the two topographs. In this region the GaAs dislocation cell structure is clearly visible. These cell walls in LEC GaAs are known to delineate small lattice tilts of 1—2 arc sec and when the dislocations climb into linear arrays (lineage features) they have been shown to form tilt boundaries of 10 100 arc sec [9,10]. The lineage features shown on the GaAs topograph (marked M) can be seen to correspond to the black-white-black lines of contrast in the CMT epilayer. Also enlargements of the box (ii) regions in figs. 4a and 4b show a good correlation between the GaAs cell structure and the black—white—black —~
578
AM. Keir et a!. / HRXRD studies of Cd~,Hg,- ,,Te/ CdTe epiraxial layers grown by MOVPE on GaAs
contrast features in the CMT layer. These features correspond to orientation contrast, where a lattice tilt runs along the line direction of the contrast line. This results in two regions of enhanced intensity sandwiching one of reduced diffracted intensity. This observation is surprising since it indicates that even in this large lattice mismatched system the substrate defect structure played an important role in determining that of the epilayer. The relatively small lattice tilts present in the GaAs substrates are magnified up to several hundred arc seconds in the CMT epilayer and seem to be the primary cause of rocking curve broadening.
the substrate preparation process, is at this moment unclear and forms the basis of further studies. Whatever the case, the interface between GaAs and CdTe is of crucial importance in determining the defect structure in the CMT epilayer. Crown Copyright © HMSO, London, 1989.
References [1] in: A.T. Semiconductor-Based Macrander, R.D. Dupuis, J.C. Bean and J.M. Brown, Heterostructures; Interfacial Structure and Stability, Eds. M.L. Green, H.W. Deckman, J.E.E. Bagin, W. Mayo, G.Y. Chin and D. Narasinham (Metallurgical Society, 1986).
4. Conclusions In this study we have shown that for CMT/ CdTe layers grown on GaAs substrates the rocking curve width /3 is spacially inhomogeneous and varies as a function of X-ray beam direction. This phenomenon is shown to be linked with large amsotropic lattice tilts present in both CdTe buffer and CMT epilayer. Although the overall defect density in CMT layers grown onto GaAs substrates is lower than in the underlying CdTe buffer, a strong relationship exists between the quality of the two. In addition, whilst the structural quality of the GaAs substrate is higher than the best epitaxial CMT layers, and the lattice mismatch is large, it has been shown here that some important defects in the CMT layer can be linked directly to the dislocation distribution in the GaAs substrate. Whether this is an intrinsic property of the substrate/layer interface, or whether it is induced by
[21 G.T. Brown, A.M. Keir, J. Giess. J.S. Gough and S.J.C Irvine, in: Microscopy of Semiconducting Materials 1989, Inst. Phys. Conf. Ser. 100, Eds. A.G. Cullis and J.L. Hutchison (Inst. Phys., London—Bristol, 1989). [3] A.R. Lang, J. AppI. Phys. 29 (1958) 597. [4] J. Tunmciffe, S.J.C. Irvine, O.D. Dosser and J.B. Mullin, J. Crystal Growth 68 (1985) 245. [5] J. Giess, J.S. Gough, S.J.C. Irvine, J.B. Mullin and G.W. Blackmore, Mater. Res. Soc. Symp. Proc. 90 (1987) 389. [6] E.L. Polisar, N.M. Boinikh, G.V. Indenbaum, A.V. Vonyakov and V.P. Schastlivi, Izv. Vyshch. Ucheb. Zaved. Fix. 6 (1968) 81. [7] G.T. Brown, A.M. Keir, M.J. Gibbs, J. Giess, S.J.C. Irvine and M.G. Astles, in: Proc. Electrochem. Soc. Symp. on Heteroepitaxial Approaches in Semiconductors, Electrochem. Soc. Fall Meeting, Chicago, Oct. 1988. [8] S.J.C. Irvine, J.S. Gough, J. Giess, M.J. Gibbs, A. Royle, CA. Taylor, G.T. Brown, A.M. Keir and J.B. Mullin, J. Vacuum Sci. Technol. A7 (1989) 285. [9] G.T. Brown, M.S. G.R. B.K. TannerIll-V and S.J. Barnett, in: Skolmck, Proc. Conf. onJones, Semi-Insulating Materials, Kab-nee-ta, 1984, Eds. D.C. Look and J.S. Blakemore (Shiva, Nantwich, 1984). [10] S.J. Barnett, PhD Thesis, University of Durham (1987).
Journal of Crystal Growth 101 (1990) 572—578 North-Holland
HIGH RESOLUTION X-RAY DIFFRACTION STUDIES OF Cd~Hg1 ~Te/CdTe EPITAXIAL LAYERS GROWN BY MOVPE ON GaAs SUBSTRATES A.M. KEIR, A. GRAHAM, S.J. BARNETI’, J. GIESS, M.G. ASTLES and S.J.C. IRVINE Royal Signals and Radar Establishment, St. Andrews Road, Great Ma/oem, Worcs. WRJ4 3PS, UK
We have applied higb resolution X-ray diffractometry and topography techniques to investigate both the lateral uniformity and structural properties of Cd~Hg1_~Telayers grown by MOVPE onto CdTe buffer layers on GaAs. On samples — 1—2 cm square, maps of rocking curve width (/3) have shown values varying from 60 arc sec (comparable to the best reported) to over 1000 arc sec on the same slice, indicating the superior value of mapping over single point measurements on this material. A good correlation has been observed between rocking curve widths, lattice tilts and the density of pyramid-like surface defects, the last of which are also associated with an increased twin density. However, on rotating the sample about its surface normal, the 400 surface symmetric /3-value varies by up to an order of magnitude, indicating that lattice tilts play an important role in broadening the rocking curve. X-ray topography reveals large tilt boundaries in the CMT epilayer which correlate with the dislocation structure in the GaAs substrate.
1. Introduction Cd~Hg1_~Te(CMT) is an important material for detection of infra-red radiation in the 8—14 ,~mwaveband. To make possible focal-plane detector arrays of large area, epitaxial CMT with high quality and uniformity in both structure and composition is required. In our laboratories two prime candidate materials have emerged for the choice of substrate. CdTe has relatively poor structural quality and uniformity, is difficult to prepare with defect-free clean surfaces and is expensive. GaAs is much cheaper, it is of higher structural quality and uniformity and its surfaces can be prepared to a higher degree of cleanliness and perfection. The main disadvantage is the very high (15%) lattice mismatch with CMT compared to 0.3% for a CdTe substrate. Despite this latter point we have grown thick, high quality epitaxial CMT layers onto GaAs substrates and in this study these layers have been assessed using double crystal X-ray diffractometry where the FWHM (/3) has been used to describe their structural quality [1]. In assessing CMT epitaxial layers we have measured many /3-values on a two-dimensional array of points over each layer surface. Since many 0022-0248/90/$03.50 © 1990
—
CMT epilayers show large lateral variations in quality, these maps of /3 give a much more meaningful characterisation than a single point measurement. Uniformity of layers can be easily quantified from such maps. In addition, relationships have been investigated between /3-values and other properties, such as the density of surface defects, which are known to have a detrimental effect on device operation, and changes in crystal orientation, which have been found to be particularly large and anisotropic for CMT layers on GaAs [2]. In this study, a layer has been profiled by chemical etching to provide the X-ray data in three dimensions and investigate the origin of these important defects.
2. Experimental The double crystal X-ray rocking curves were measured on a Bede 300 diffractometer equipped with an X— Y sample stage. Control software for the X— Y stage, developed in-house, enabled rocking curve width (/3), peak height and relative angular displacement (e’) to be mapped automatically over an area of sample up to 75 x 75 mm. The X-ray source was a GX21 rotating anode
Elsevier Science Publishers B.V. (North-Holland)
A.M Keir eta!.
/ HRXRD
studies of Cd~ Hg
1 - ~Te/ CdTe epitaxial layers grown by MOVPE on GaAs
generator with a Cu target running at 2.8 kW. The reference crystal was 100 InSb and the 400 reflection was used for both the reference crystal and the sample in the + non-dispersive diffraction geometry. Beam size at the sample was 0.5 mm (X) by 1 mm (Y). The rocking curve data were mapped at 1.5 mm intervals in both X and Y directions. The theoretical /3 value for these diifraction conditions of the CdTe layers was calculated to be 14 arc sec and for Cd02Hg08Te was 23 arc sec. The X-ray extinction distance for the 400 reflection is 3.5 ~.tm for CdTe and 2.8 ~tm for Cd02Hg08Te layers. Lang topography [3] was used to examine the CMT layer and also the GaAs substrate after chemical etching had removed the CMT. The 511 reflection (extinction distance 5.5 ~iLm)was used for CMT epilayer and the 620 reflection for the GaAs substrate (extinction distance 9.7 ~tm). On all the diffraction data maps and topographs shown herein the arrow marked X indicates the direction of the incident X-ray beam projected onto the plane of the sample surface. The CMT layers were grown in a horizontal MOVPE reactor
using dimethylcadmium (Me2Cd) and diisopropyltelluride (Pr~Te)with a liquid Hg source at a vapour pressure of 0.01 atm. The layers were grown by the interdiffused multilayer process (IMP) whereby the gas flows are switched between conditions optimised for HgTe and CdTe growth [4]. 100 cycles of HgTe/CdTe interdiffused at the growth temp of 350°Cmade up the 16 p~mCMT layer analysed in this paper. Growth rates were 10.6 ~tm/h and 10.0 ~tm/h for HgTe and CdTe, respectively. The GaAs substrate was 2° off (100) towards (101). A CdTe buffer layer of 4 j.Lm thickness was grown between the substrate and CMT layer in order to prevent Ga diffusing into the CMT and to isolate the highly dislocated GaAs/CdTe interface [5]. Free etching of CMT layers was achieved using a 1% bromine/methanol etch (etch rate 4 ~.tm/min) and thicknesses were determined from etch calibration experiments based on masking and measuring step heights. Etch pit densities for the CMT layer were determined using the Polisar etch [6]. The total depth of layer removed between each set of X-ray measurements was 4 p~m.
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Fig. 1. Maps of the /3 values for the 400 reflection. Maps (a) and (b) are taken from the as-grown CMT epilayer surface and maps (c) and (d) of the CdTe buffer after chemical removal of the CMT epilayer. The maps cover an area 15 x 9 mm in the centre of the sample (the full extent of the sample was 23 x 12 mm).
574
AM. Keir et a!.
/ HRXRD studies of Cd~,Hg1
~Te/ CdTe epitaxial layers grown by MOVPE on GaAs
3. Results and discussion
1000 D
Figs. la and lb show maps of rocking curve width (/3) for the sample before etching with the CMT layer at its as-grown thickness of 16 ~tm. These are recorded with the diffraction plane in orthogonal directions as indicated by the arrow, X. A well-defined region in which /3 is larger than for the remainder of the sample can be seen in both maps. In map (a) the minimum value of /3 is 91 arc sec and the maximum 225 arc sec. In map (b), /3 is greater at every corresponding point with a minimum of 152 arc sec and a maximum of 1230 arc sec. Figs. ic and id show the same area of the sample after chemical removal of the CMT to leave approximately 3 ~om of the CdTe buffer layer. Note that, although the scale is different to cover a range of higher /3 values, the general pattern is very similar to figs. la and lb. (The slightly higher values towards right and left are due to the CdTe layer being thinner after uneven etching.) Fig. 2 shows graphically the relationship between /3 values at equivalent points in the CdTe buffer layer and the CMT epilayer. Although the
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Fig. 2. Graph of the CMT epilayer rocking curve width versus that of the buffer layer. The circles are values taken from figs. la and ic and the squares values taken from figs. lb and ld. All plotted values are averages of four adjacent data points from fig. 1.
slope of the curve is much less than unity (indicating a large improvement of structural quality of the CMT compared to the CdTe buffer layer), the correlation observed between the /3 values clearly demonstrates the importance of buffer layer quality in influencing the final structural quality of the CMT epilayer. /3 values recorded at intermediate
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Fig. 3. Maps of relative angular position of the 400 Bragg peak. Again maps (a) and (b) are taken from the CMT sample surface and maps (c) and (d) of the CdTe buffer layer after removal of the CMT epilayer.
AM. Keir et aL
/ HRXRD
studies of Cd~Hg,- ,Te/ CdTe epitaxial layers grown by MOVPE on GaAs
4 ~tm depth intervals in the CMT layer follow a very similar pattern to that reported in previous papers [7,8] (for both orthogonal orientations of the diffraction plane). /3 decreases most rapidly in the few microns nearest the buffer layer and tends towards a constant value with increasing layer thickness. An estimate of the compositional uniformity of the CMT epilayer made by infra-red transmission measurements showed a maximum x variation of ±0.0015 over 1 cm. This could change the Bragg angle for the 400 reflection by only 1.5 arc sec and therefore should not contribute to significant broadening of the rocking curve. Typical sample curvature can move the Bragg peak by 200 arc sec over 15 mm of the sample and is expected to broaden rocking curves by 6 arc sec [7]. Simultaneous with recording the /3 values of fig. 1, the relative angular position (0’) of the 400 diffraction peak was recorded. Maps of this parameter are given in fig. 3. As in fig. 1, figs. 3a and 3b are for the CMT and figs. 3c and 3d for the CdTe buffer. These maps display the relative —
575
orientation of the layer (100) planes in the plane of diffraction. In figs. 3a and 3c the maximum change is 200 arc sec so lattice tilts may be confused with the effects of sample curvature. For the orthogonal diffraction in figs. 3b and 3d, the much larger values can be attributed to lattice tilt, the darkest regions representing a tilt in the (100) epilayer planes of almost 2° towards the (011) substrate planes. The very strong correspondence between figs. 3b and 3d shows that all the largescale tilt features present in the CMT layer originate in the CdTe buffer layer. It is interesting to note that although the /3 values are dramatically reduced on moving from the CdTe buffer layer to the CMT epilayer the relative lattice tilt (orientation) remains constant. Returning to fig. 1, note that the greatest rocking curve broadening occurs around the edge of the triangular tilted region in figs. lb and ld. This is due to the large change in orientation of the (100) planes across the area sampled by the X-ray beam at these points. Within the roughly triangular tilted region, /3 values are greater in both orthogonal directions. —
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Fig. 4. (a) 511 reflection topograph of the CMT layer; (b) 620 reflection topograph of the GaAs substrate taken through the CdTe buffer layer (note that these show the full sample area of 23 x 12 nun, a larger area than is covered by the rocking curve maps); (c) enlargement of the boxed region (i) of (b).
576
AM. Keir et aL
/
HRXRD studies of Cd~,Hg
1 ~Te/ CdTe epitaxial layers grown by MO VPE on GaAs
b
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Fig. 4 (continued).
Etch pit density measurements also show higher values in this region. At the surface of the 16 j.tm CMT layer, values wereand approximately 3 X 2 in EPD the tilted region 6 x 106 cm2 l0~ cm for the majority of the sample surface. Since EPD values are believed to be proportional to dislocation density, this indicates a 5-fold increase in dislocation density for the tilted region with respect to the rest of the sample. —
Fig. 4a shows a Lang topograph of the CMT layer at 16 ~tm thick. Since all the large lattice tilts in the sample are sample perpendicular diffraction plane, the whole remainstointhecontrast but the diffracted X-rays from the tilted regions are displaced in the vertical plane. The large triangular tilted region can be seen clearly displaced towards the bottom of the figure. Laue diffraction patterns were taken to de-
A.M. Keir et aL
a
/
HRXRD studies of Cd~,,Hg
1— ,,Te/ CdTe epitaxial layers grown by MO VPE on GaAs
577
b
IL
Fig. 5. (a) Enlargement of the boxed region (ii) in fig. 4a. (b) Enlargement of the boxed region (ii) in fig. 4b.
termine the relative orientation of the epilayer with respect to the substrate, which was cut 2°off (100) towards the (101) lattice planes. It was found that, over the whole sample, the epilayer [011] direction lies parallel to the substrate [011] direction (both are, in fact, inclined at 1.4° to the sample surface). In the orthogonal direction, over most of the sample surface, epilayer and substrate [011] directions are inclined at 1.4° (in the (011) plane) to one another, ie the epilayer [011] direction lies in the plane of the sample surface. However, in the triangular region of high /3 values the epilayer [011] direction is tilted by a further 2° away from the substrate [011] direction. This observation relates to the amsotropic nature of the /3 values in that the largest values are observed when the relative lattice tilt between epilayer and substrate lies in the plane of diffraction. Fig. 4b shows a 620 reflection topograph of the GaAs substrate taken through the remains of the CdTe buffer layer. This topograph provides some important insight into the nature and origin of both the pyramid-like surface features and the large non-uniform lattice tilts measured in the CMT epilayer. Fig. ‘Ic shows an enlargement of
the boxed area (i) in fig. 4b. The white spots on this topograph correspond to the sites of the pyramid-like surface defects common to CMT epilayers. (In a previous paper [2] we reported on the link between these pyramids and twin density.) The majority of these spots can be seen to lie on the highly dislocated cell walls in the GaAs substrate rather than in the dislocation-sparse cell interior. Fig. 5 shows enlargements of the boxed regions (ii) of figs. 4a and 4b. Note that the horizontal scales do not match due to the different reflections used for the two topographs. In this region the GaAs dislocation cell structure is clearly visible. These cell walls in LEC GaAs are known to delineate small lattice tilts of 1—2 arc sec and when the dislocations climb into linear arrays (lineage features) they have been shown to form tilt boundaries of 10 100 arc sec [9,10]. The lineage features shown on the GaAs topograph (marked M) can be seen to correspond to the black-white-black lines of contrast in the CMT epilayer. Also enlargements of the box (ii) regions in figs. 4a and 4b show a good correlation between the GaAs cell structure and the black—white—black —~
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AM. Keir et a!. / HRXRD studies of Cd~,Hg,- ,,Te/ CdTe epiraxial layers grown by MOVPE on GaAs
contrast features in the CMT layer. These features correspond to orientation contrast, where a lattice tilt runs along the line direction of the contrast line. This results in two regions of enhanced intensity sandwiching one of reduced diffracted intensity. This observation is surprising since it indicates that even in this large lattice mismatched system the substrate defect structure played an important role in determining that of the epilayer. The relatively small lattice tilts present in the GaAs substrates are magnified up to several hundred arc seconds in the CMT epilayer and seem to be the primary cause of rocking curve broadening.
the substrate preparation process, is at this moment unclear and forms the basis of further studies. Whatever the case, the interface between GaAs and CdTe is of crucial importance in determining the defect structure in the CMT epilayer. Crown Copyright © HMSO, London, 1989.
References [1] in: A.T. Semiconductor-Based Macrander, R.D. Dupuis, J.C. Bean and J.M. Brown, Heterostructures; Interfacial Structure and Stability, Eds. M.L. Green, H.W. Deckman, J.E.E. Bagin, W. Mayo, G.Y. Chin and D. Narasinham (Metallurgical Society, 1986).
4. Conclusions In this study we have shown that for CMT/ CdTe layers grown on GaAs substrates the rocking curve width /3 is spacially inhomogeneous and varies as a function of X-ray beam direction. This phenomenon is shown to be linked with large amsotropic lattice tilts present in both CdTe buffer and CMT epilayer. Although the overall defect density in CMT layers grown onto GaAs substrates is lower than in the underlying CdTe buffer, a strong relationship exists between the quality of the two. In addition, whilst the structural quality of the GaAs substrate is higher than the best epitaxial CMT layers, and the lattice mismatch is large, it has been shown here that some important defects in the CMT layer can be linked directly to the dislocation distribution in the GaAs substrate. Whether this is an intrinsic property of the substrate/layer interface, or whether it is induced by
[21 G.T. Brown, A.M. Keir, J. Giess. J.S. Gough and S.J.C Irvine, in: Microscopy of Semiconducting Materials 1989, Inst. Phys. Conf. Ser. 100, Eds. A.G. Cullis and J.L. Hutchison (Inst. Phys., London—Bristol, 1989). [3] A.R. Lang, J. AppI. Phys. 29 (1958) 597. [4] J. Tunmciffe, S.J.C. Irvine, O.D. Dosser and J.B. Mullin, J. Crystal Growth 68 (1985) 245. [5] J. Giess, J.S. Gough, S.J.C. Irvine, J.B. Mullin and G.W. Blackmore, Mater. Res. Soc. Symp. Proc. 90 (1987) 389. [6] E.L. Polisar, N.M. Boinikh, G.V. Indenbaum, A.V. Vonyakov and V.P. Schastlivi, Izv. Vyshch. Ucheb. Zaved. Fix. 6 (1968) 81. [7] G.T. Brown, A.M. Keir, M.J. Gibbs, J. Giess, S.J.C. Irvine and M.G. Astles, in: Proc. Electrochem. Soc. Symp. on Heteroepitaxial Approaches in Semiconductors, Electrochem. Soc. Fall Meeting, Chicago, Oct. 1988. [8] S.J.C. Irvine, J.S. Gough, J. Giess, M.J. Gibbs, A. Royle, CA. Taylor, G.T. Brown, A.M. Keir and J.B. Mullin, J. Vacuum Sci. Technol. A7 (1989) 285. [9] G.T. Brown, M.S. G.R. B.K. TannerIll-V and S.J. Barnett, in: Skolmck, Proc. Conf. onJones, Semi-Insulating Materials, Kab-nee-ta, 1984, Eds. D.C. Look and J.S. Blakemore (Shiva, Nantwich, 1984). [10] S.J. Barnett, PhD Thesis, University of Durham (1987).